Trojan detected can't see desktop/start

Hello,
When my computer reboots it goes only as far as my user acct. I click on my acct and for a brief second I see my desktop (no icons) and then it goes back to the user acct and it is saying "logging off". I click on user acct again & it says "save settings" "logging off". So I do not know whether my boot sector is corrupt or whether it is a registry problem. Before all this started happening, my system detected a trojan. I acted quickly to fix it but it may have already reinvented inself and did the damage. What could I try before having to resort to a Computer Tech? My computer is a Dell Inspiron 531S Help!